The present invention is generally concerned with mobile radiocommunication systems.
The present invention is more particularly concerned with power control techniques used in such systems to improve performances (in terms of quality of service, of capacity, . . . etc.).
The present invention is in particular applicable to mobile radiocommunication systems of CDMA (xe2x80x9cCode Division Multiple Accessxe2x80x9d) type. In particular, the present invention is applicable to UMTS (xe2x80x9cUniversal Mobile Telecommunication Systemxe2x80x9d).
As is known, CDMA systems use two types of power control techniques, a so-called open-loop power control technique, and a so-called closed loop power control technique (also called hereinafter CLPC). These power control techniques may be recalled for example for the uplink transmission direction, i.e. from MS (xe2x80x9cMobile Stationxe2x80x9d) to BTS (xe2x80x9cBase Transceiver Stationxe2x80x9d). In the open-loop power control, a MS transmit power is controlled based on the power received by this MS from a BTS. In the CLPC, a MS transmit power is controlled based on the transmission quality of the link between this MS and a BTS, as estimated at this BTS.
The transmission quality of a link between a MS and a BTS depends on the ratio of the received signal power and the interference power, also called SIR (Signal-to-Interference Ratio). When the SIR of a MS is low, or equivalently when the powers of the other MSs are much higher than its power, its performances dramatically decrease. The CLPC algorithm enables to keep the SIR of each user as close as possible to the target SIR.
The principle of the CLPC algorithm is that the BTS periodically estimates the SIR of the received signal from each MS, and compares this estimated SIR to a target SIR (SIRtarget). If the estimated SIR is lower than the target SIR, the BTS sends a power control command to the MS, for the MS to increase its transmit power. Otherwise, the BTS sends a power control command to the MS, for the MS to decrease its transmit power. The target SIR is chosen by the BTS as a function of the required quality of service.
To be efficient and track the variations of the SIR as closely as possible, in particular in fast changing environments, the CLPC needs to be rapid; as all example, in third generation systems like for example UMTS, power control commands are typically sent to a MS every slot in a frame (a slot being an elementary time unit in a data unit, or frame, transmitted in such a system, the frame duration being typically equal to 10 ms, and the slot duration to {fraction (1/15)} of the frame duration).
Now there are some situations in mobile radiocommunication systems, in which transmission of power control commands has to be momentarily interrupted. This will also hereinafter be referred to as transmission interruptions. This is also referred to as xe2x80x9ctransmission gapsxe2x80x9d in UMTS system for example.
As an example, in CDMA systems, downlink transmission from a BTS to a MS may momentarily be interrupted to allow this MS to make measurements on frequencies other than the one used for this downlink transmission (in particular for handover preparation purposes, in particular inter-frequency handovers). Such a transmission mode including transmission interruptions is also referred to as xe2x80x9cslotted modexe2x80x9d or xe2x80x9ccompressed modexe2x80x9d in UMTS system for example. A transmission interruption may last several slots (typically up to 15 slots, i.e. one frame, in downlink compressed mode). During these transmission interruptions, the CLPC is interrupted. Thus, the BTS does not send any more power control commands to the MS, and the uplink signals from this MS are no longer power controlled. Uplink transmission may simultaneously be interrupted, but in either case, as a result, the efficiency of the CLPC is significantly decreased, and the performances of the system may be seriously degraded.
In European patent application no. 99400894.4 filed on Apr. 12, 1999 by Applicant, a solution has been proposed to avoid such degradations of performances, due to such transmission interruptions.
Essentially, according to this prior patent application, when transmission is resumed after a transmission interruption, said power control algorithm is implemented with at least one modified parameter, for a given duration, said at least one modified parameter and said given duration being determined so as to compensate for the effects of said transmission interruption on power control.
According to a first embodiment disclosed in this prior patent application, said given duration has a predetermined value.
According to a second embodiment disclosed in this prior patent application, said given duration is determined to be passed when a given condition is fulfilled, based for example on successive power control results with said at least one modified parameter, for example when two consecutive power control commands obtained with said at least one modified parameter are opposed.
This second embodiment thus has the advantage that this given duration, or xe2x80x9crecovery periodxe2x80x9d, is not fixed and can be adapted to each situation, depending for example on mobile speed, on environment, . . . etc. However, adaptive algorithms are usually very sensitive to errors (which in this context include SIR estimation errors, power control commands errors, . . . ). Thus, with an adaptive compensation length, there is a risk that the recovery period be too short, because of such errors.
On the other hand, a fixed recovery period offers the advantage to be robust to errors, but is not optimal, since it cannot be adapted to each situation.
The present invention enables to take profit of both types of algorithms, i.e. offers the advantage of an adaptive algorithm having a lower sensitivity to errors, or of a fixed algorithm having more flexibility.
An object of the present invention is therefore a method for improving performances of a mobile radiocommunication system using a power control algorithm and being subject to transmission interruptions, a method wherein a recovery period is provided following a transmission interruption, to compensate for the effects of said transmission interruption on said power control algorithm, and wherein said recovery period includes different parts wherein different types of compensation are successively carried out, said types being determined so as to ensure that a minimum compensation can first be obtained, and that compensation is not higher than necessary thereafter.
According to another object of this invention, said recovery period includes a first part having a length TMIN wherein a fixed-length compensation is carried out, followed by a second part having a maximum length TMAXxe2x88x92TMIN wherein an adaptive-length compensation is carried out, TMIN being the minimum compensation length inside said recovery period, and TMAX being the maximum compensation length inside said recovery period.
According to another object of the present invention, said first and second parts each have a length equal to half of the length of a transmission interruption period.
According to another object of the present invention, said recovery period includes N segments, each segment xe2x80x9cnxe2x80x9d including a first part having a length Tminn wherein a fixed-length compensation is carried out, followed by a second part having a maximum length Tmaxnxe2x88x92Tminn wherein an adaptive-length compensation is carried out, Tminn being the minimum compensation length inside segment xe2x80x9cnxe2x80x9d, and Tmaxn being the maximum compensation length inside segment xe2x80x9cnxe2x80x9d.
According to another object of this invention, parameters defining said parts of a recovery period and/or said types of compensation may vary from one recovery period to another one.
According to another object of this invention, parameters defining said parts of a segment and/or said types of compensation may vary from one segment to another one.
According to another object of the present invention, said compensation being obtained by modifying at least one parameter of said power control algorithm during said recovery period, said at least one modified parameter has a value which varies from one segment to another one inside said recovery period, in the sense of a lower compensation from the beginning to the end of this recovery period.
According to another object of this invention, said at least one modified parameter having a value which varies from one segment to another one is an increased power control step size having a value which decreases from one segment to a next one inside said recovery period.
According to another object of the present invention, said minimum length Tminn is provided equal to said maximum length Tmaxn.
According to another object of the invention, said power control is performed in the uplink transmission direction of said mobile radiocommunication system.
According to another object of the invention, said power control is performed in the downlink transmission direction of said mobile radiocommunication system.
According to another object of the invention, said mobile radiocommunication system is of CDMA type.
Another object of the present invention is a mobile radiocommunication system including two entities involved in a power control algorithm, and wherein means are provided in one of said entities for signalling to the other entity parameters defining said parts of a recovery period and/or said types of compensation to be carried out in said parts, for performing such a method.
According to another object of this invention, one of said two entities is a mobile radiocommunication network entity.
According to another object of this invention, one of said two entities is a mobile station.
Another object of the present invention is a mobile radiocommunication network entity (such as in particular BTS) for performing such a method.
Another object of the present invention is a mobile station (MS) for performing such a method.
According to another object of the invention, a mobile radiocommunication network entity comprises, for performing said method in said uplink transmission direction:
means for performing an uplink power control algorithm, and for compensating for the effects of transmission interruptions on said power control algorithm, according to said method,
means for sending corresponding power control commands to a mobile station.
According to another object of the invention, a mobile station comprises, for performing said method in said uplink transmission direction:
means for receiving said power control commands from a mobile radiocommunication network entity.
According to another object of the invention, a mobile radiocommunication network entity comprises, for performing said method in said uplink transmission direction:
means for implementing an uplink power control algorithm,
means for sending corresponding power control commands to a mobile station.
According to another object of the invention, a mobile station comprises, for performing said method in said uplink transmission direction:
means for receiving said power control commands from a mobile radiocommunication network entity, and for modifying said power control commands, so as to compensate for the effects of transmission interruptions on said power control algorithm, according to said method.
According to another object of the invention, a mobile station comprises, for performing said method in said downlink transmission direction:
means for performing a downlink power control algorithm, and for compensating for the effects of transmission interruptions on said power control algorithm, according to said method,
means for sending corresponding power control commands to a mobile radiocommunication network entity.
According to another object of the invention, a mobile radiocommunication network entity, comprises, for performing said method in said downlink transmission direction:
means for receiving said power control commands from a mobile station.
According to another object of the invention, a mobile station comprises, for performing said method in said downlink transmission direction:
means for implementing a downlink power control algorithm,
means for sending corresponding power control commands to a mobile radiocommunication network entity.
According to another object of the invention, a mobile radiocommunication network entity comprises, for performing said method in said downlink transmission direction:
means for receiving said power control commands from a mobile station, and for modifying said power control commands so as to compensate for the effects of transmission interruptions on said power control algorithm, according to said method.